Conventional subscriber networks are constructed by an asymmetric digital subscriber line (ADSL) or very-high-speed digital subscriber line (VDSL) using a telephone line, or a cable modem using a coaxial cable. Because the above-mentioned structure uses a copper wire, its capacity is limited to a maximum of 52 Mbps according to a transmission distance. When an increased amount of data traffic, and various multimedia services such as high definition television (HDTV), e-commerce, video on demand (VOD), etc. are taken into account, capacity of existing subscriber networks needs to be extended. To extend the network capacity, a large amount of research is being performed on a wavelength division multiplexing-passive optical network (WDM-PON) system based on an optical fiber.
In the WDM-PON system, one wavelength per subscriber must be assigned. Accordingly, one light source per subscriber is required, thereby increasing economic burden on the subscriber. That is, when a distributed feedback (DFB) laser diode (LD) in which an oscillating wavelength is fixed is used as a fixed light source per subscriber, it is difficult for the DFB LD to be commercialized because the cost of the DFB LD is high. Accordingly, a key of the WDM-PON technology is to implement a low-cost light source.
There is an advantage in that the cost of a Fabry-Perot (FP) laser diode (LD) is low. However, the FP LD is not appropriate to be used as a light source in the WDM-PON system due to mode partition noise [1].
Another method for implementing the low-cost light source for the WDM-PON uses a wavelength-locked FP LD [3]. The method using the wavelength-locked FP LD injects a wavelength of an incoherent light source into the FP LD oscillating in multimode, fixes an oscillating wavelength of the FP LD to the injected wavelength, and enables the FP LD to oscillate in single mode, thereby generating a WDM light source.
For example, a light emitting diode (LED), an erbium-doped fiber amplifier (EDFA) for generating amplified spontaneous emission (ASE), or a super luminescent diode (SLD) are used as a broadband light source for wavelength injection into the wavelength-locked FP LD [2] and [3].